This invention relates to electrical motors, and more particularly to direct current electrical motors of the compound form which have one field winding connected in series with the armature winding, and another field winding connected in parallel therewith.
Most DC electrical motors have an armature disposed for rotation in the magnetic field of a stator, and include a commutator for transmitting current to the rotating armature winding. The stator magnetic field is established by one or more field windings. Motors of this general class may be subdivided into three basic types according to the nature of the interconnections between the armature windings and the field windings. The basic types are shunt motors, series motors, and compound motors, and each exhibits somewhat different operational characteristics.
A shunt motor in which the field winding is connected in parallel with the armature winding tends to produce a variable torque at a substantially constant speed and thus accommodates to variable loading conditions. In a series motor, the armature and field windings are connected in series which produces greater starting torque but causes both torque and speed to vary widely under changing loads. Compound motors have at least two field windings with one being in series with the armature winding, and the other being in parallel therewith. As a consequence, compound motors offer a compromise between the differing operational characteristics of shunt motors and series motors. A compound motor exhibits greater starting torque than a shunt motor while being less variable in speed under changing loading than in a series motor. As the relatively flat speed-torque curve is preferable in many electrical motor applications, compound motors are extensively used for driving diverse types of powered apparatus.
Considering now another aspect of electrical motors, it is well recognized that fast start-up is desirable, at least in most motor usages. Fast start-up is usually preferable not only to speed operation of the device being driven by the motor, but also to reduce the overheating and structural deterioration of the motor which tends to occur during the start-up period. In particular, an electrical motor has very little internal resistance to current flow when voltage is first applied to the motor terminals. Once the armature begins to turn, the motor inherently functions also as a generator producing a back EMF (electromotive force) that opposes and, in effect, reduces the voltage which is being applied to the motor terminals. The back EMF increases as armature rotation speeds up, and is eventually of sufficient magnitude to greatly reduce current flow through the motor. At the first instant of start-up, and for a brief period thereafter, the lack of normal back EMF allows extremely large current surges to flow through the windings and other conductors, including the brushes. These current surges during start-up cause severe heating, brush deterioration, power wastage, may interfere with proper commutation, and in general tend to reduce the operational life of the motor. If the motor is being operated from a battery, the abnormal current may also cause battery deterioration.
Thus, the working efficiency and durability of electrical motors, including compound DC motors, may be increased to the extent that faster start-up can be accomplished as this reduces the temporary periods of damagingly high current flow. This is a particularly important aspect of motor design where frequent starting and stopping of a motor is probable. The motor employed in electrohydraulic systems for controlling mast-tilting and fork-lifting functions of an industrial lift truck is one example of a motor usage where a compound motor is particularly desirable but in which frequent start-ups may be necessary, there being many diverse other examples of such motor applications known to those skilled in the art.